As information devices have rapidly come into widespread use, a wide variety of types of information has come to be stored in information devices in recent years. Accordingly, a very important issue is to establish a security technology that suppresses, for example, information from leaking from information devices. In view of the above situation, various authentication technologies have been proposed and are being implemented. Authentication technology is technology that uses, for example, password authentication, biometric authentication, card authentication, or the like. However, many existing authentication technologies carry out authentication only at login, so they have the common problem that if, for example, a third party has unauthorized access to an information device while a legitimate user is away from the location at which the information device is installed, the unauthorized access is difficult to detect.
To address the above problem, continuous authentication technologies have been proposed in which the user is subject to continuous authentication even after the user has logged in K. Niinuma, U. Park, A. K. Jain, “Soft Biometric Traits for Continuous User Authentication”, IEEE Transactions on Information Forensics and Security, Vol. 5, No. 4, pp. 771-780, 2010, P. Viola and M. Jones, “Rapid object detection using a boosted cascade of simple features”, in Proc. IEEE Computer Vision and Pattern Recognition, pp. I—511-I—518, 2001 and B. D. Lucas and T. Kanade, “An iterative image registration technique with an application to stereo vision”, in Proc. 7th IJCAI, Vancouver, B. C., Canada, pp. 674-679, 1981.
Color histogram authentication, which is a continuous authentication technology that uses color histograms of an image on which a user is pictured, has the advantage that robust continuous authentication is possible for changes in the orientation of the user in comparison with face authentication and other methods in which key strokes or the like is used. In color histogram authentication, the color histogram of an area associated with a user on an image is registered and the user-associated area on an image is detected according to color histogram similarity with the registered color histogram, for example. Color histogram enables continuous authentication by tracking the movement of the user-associated area.